Synthesis of C-substituted t-BuNH-8,9-R,R′-nido-7,8,9-C 3B8H9 (R,R′ = H,H; MeH; Me,Me; Ph,H and Ph,Ph) tricarbollide compounds and their tautomeric conversions. Effect of substituents on tautomeric equilibria between neutral and zwitterionic forms

Article abstract of DOI:10.1039/b924754h

Treatment of C-substituted nido dicarbadecaboranes 5,6-R′,R-5,6- C₂B₈H₁₀ (1) (where R′,R = H,H (1a); H,Me, (1b); Me,Me, (1c); H,Ph, (1d) and Ph,Ph, (1e) with 1,8-bis-(dimethylamino) naphthalene (proton sponge = PS) and t-BuNC in CH₂Cl₂, followed by acidification, generated a series of pure neutral compounds 7-t-BuNH-8,9-R,R′-nido-7,8,9-C₃B₈H₉ (N2) (where R,R′ = H,H (N2a); H,Me (N2b); Me,Me (N2c); H,Ph (N2d), and Ph,Ph (N2e)), each of which exhibits tautomerism. Dissolution of the substituted compounds (N2b-N2e) in protic solvents (PRS), such as MeCN and Me₂CO, leads to tautomeric equilibrium with the zwitterionic tautomers 7-t-BuNH ₂-8,9-R,R′-nido-7,8,9-C₃B₈H₈ (Z2) (where R,R′= H,H (Z2a); H,Me (Z2b); Me,Me (Z2c); H,Ph (Z2d) and Ph,Ph (Z2e)), while the unsubstituted compound N2a exhibits absolute tautomerism - a complete conversion into the zwitterionic tautomer Z2a. The tautomeric behaviour of individual compounds is therefore strongly affected by the nature of the substituent, as assessed via NMR spectroscopy in terms of tautomerisation constants K_T = C_Z2/C_N2 (where C_Z2 and C_N2 are equilibrium concentrations of Z2 and N2 forms in a given solvent). Individual tautomers were characterised by ¹¹B and ¹H NMR spectroscopy and the structure of the monomethylated N2b tautomer was determined by an X-ray diffraction study.

Full text of DOI:10.1039/b924754h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis of C-substituted t-BuNH-8,9-R,R¢-nido-7,8,9-C3B8H9 (R,R¢ =
H,H; MeH; Me,Me; Ph,H and Ph,Ph) tricarbollide compounds and their
tautomeric conversions. Effect of substituents on tautomeric equilibria
between neutral and zwitterionic forms†
a
a
a
b
ˇ
Mario Bakardjiev, Josef Holub, Bohumil St´ıbr* and Ivana C´ısarˇova´
Received 24th November 2009, Accepted 17th February 2010
First published as an Advance Article on the web 24th March 2010
DOI: 10.1039/b924754h
Treatment of C-substituted nido dicarbadecaboranes 5,6-R¢,R-5,6-C2B8H10 (1) (where R¢,R = H,H (1a);
H,Me, (1b); Me,Me, (1c); H,Ph, (1d) and Ph,Ph, (1e) with 1,8-bis-(dimethylamino)naphthalene (proton
sponge = PS) and t-BuNC in CH2Cl2, followed by acidification, generated a series of pure neutral
compounds 7-t-BuNH-8,9-R,R¢-nido-7,8,9-C3B8H9 (N2) (where R,R¢ = H,H (N2a); H,Me (N2b);
Me,Me (N2c); H,Ph (N2d), and Ph,Ph (N2e)), each of which exhibits tautomerism. Dissolution of the
substituted compounds (N2b–N2e) in protic solvents (PRS), such as MeCN and Me2CO, leads to
tautomeric equilibrium with the zwitterionic tautomers 7-t-BuNH2-8,9-R,R¢-nido-7,8,9-C3B8H8 (Z2)
(where R,R¢= H,H (Z2a); H,Me (Z2b); Me,Me (Z2c); H,Ph (Z2d) and Ph,Ph (Z2e)), while the
unsubstituted compound N2a exhibits absolute tautomerism – a complete conversion into the
zwitterionic tautomer Z2a. The tautomeric behaviour of individual compounds is therefore strongly
affected by the nature of the substituent, as assessed via NMR spectroscopy in terms of tautomerisation
constants KT = CZ2/CN2 (where CZ2 and CN2 are equilibrium concentrations of Z2 and N2 forms in a
given solvent). Individual tautomers were characterised by 11B and 1H NMR spectroscopy and the
structure of the monomethylated N2b tautomer was determined by an X-ray diffraction study.
Introduction
Tautomerism, which is an equilibrium between two or more
alternative structures of a single species, has been observed for
many compounds in organic chemistry. This phenomenon is
typical for compounds that contain a functionality that is able
to donate a proton, and another, which is able to accept it, within
the same molecule and in close enough proximity to one another.
The tautomerisation equilibrium is in all such cases transmitted
via a common anion. As far as we are aware, the most pronounced
textbook example of tautomerism is the equilibrium between the
keto and enol forms of acetylacetone, as shown in Scheme 1.1 The
tautomerisation constant, defined as KT = Cenol/Cketo, is in this
case 3.6 for neat acetylacetone. As a consequence of tautomeric
equilibria no pure tautomers of acetylacetone can be isolated.
On the other hand, quite common in organic chemistry is the
case that a compound occurs in only one tautomeric form. For
Scheme 1 Equilibrium between keto and enol forms in acetylacetone.
example, malonamide, NH2CO–CH2–CONH2, is known as a keto
form only,2 while hexafluoroacetylacetone occurs only as the enol
3
called absolute tautomerism has been reported for the first time
by our group in a preliminary communication.4
=
form, F3CCO–CH C(OH)CF3. As far as we are aware, a third
case when both tautomeric forms can be isolated in pure state, so
This unique phenomenon was established in the C-amino
substituted tricarbollide (11-vertex nido-tricarbaborane) series in
which a compound adopts only one of two different tautomeric
structures, either 7-R,R¢NH-7,8,9-C3B8H10 (Z) (zwitterionic) or
7-R,R¢N-7,8,9-C3B8H11 (N) (neutral) (where R,R¢ = hydrogen or
alkyls). Each tautomer can be interconverted using solvents of
different proton-transfer capability. We have also established that
the solvent induced Z → N and N → Z conversions proceed
via common [7-R,R¢N-7,8,9-C3B8H10]- anions and are essentially
aInstitute of Inorganic Chemistry, Academy of Sciences of the Czech
ˇ
Republic, CZ-250 68 Rezˇ, The Czech Republic. E-mail: stibr@iic.cas.cz;
Fax: +420 2 2094 1502; Tel: +420 2 6617 3106
bDepartment of Chemistry, Faculty of Natural Science, Charles University,
Albertov 6, 128 40 Prague 2, The Czech Republic. E-mail: cisarova@
natur.cuni.cz; Tel: +420 2 21951249
† CCDC reference number 753108. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b924754h
4186 | Dalton Trans., 2010, 39, 4186–4190
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independent of R,R¢ alkyls attached to the nitrogen atom.4 On the
other hand, some compounds of the same cluster constitution,
but substituted on boron, such as 10-Me-7-t-BuNMe-7,8,9-
C3B8H10 and 10,11-Me2-7-t-BuNMe-7,8,9-C3B8H9, have so far
been isolated only in the N form.5 This means that substituents on
boron rather than on carbon vertices of the carborane cluster may
significantly affect the formation of individual tautomers. In this
work, which initiates more detailed studies of the effects of cluster
substitution on tautomeric behaviour, we would like to report on
the synthesis of the mutually tautomeric derivatives 7-t-BuNH-
8,9-R,R¢-nido-7,8,9-C3B8H9 and 7-t-BuNH2-8,9-R,R¢-nido-7,8,9-
C3B8H8 that contain H, Me and Ph substituents on cluster carbon
atoms, in close proximity to proton donating centres. Presented
will also be scope and limitation of their room-temperature tau-
tomeric interconversions along with crystallographic and NMR
(11B and 1H) characterisation of individual tautomeric pairs.
Table 1 Tautomerisation constants K (% of conversion to the Z2 form)
for 8,9-R,R¢-substituted compounds of type 2 in common NMR solvents
at 296 K
Compounda
CD3CN
(CD3)2CO
CDCl3
CD2Cl2
2a
2b
2c
2d
2e
• (100)
0 (0)
0 (0)
0.55 (35)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
5.62 (85)
2.02 (67)
19.00 (95)
25.00 (96)
2.91 (75)
1.39 (58)
15.00 (93)
17.00 (94)
a For compounds with KT = 0 and infinity, pure (absolute) N2 or Z2
tautomers can be isolated.
It should be noted that crystallisation from aqueous EtOH also
strongly enhances the formation of Z2 tautomers.
As seen in Fig. 1, a simple 11B NMR experiment at 296 K can be
designed to clearly demonstrate and understand the tautomeric in-
terconversion for the simplest (unsubstituted) tautomeric pair 7-t-
BuNH-7,8,9-C3B8H11 (N2a) and 7-t-BuNH2-7,8,9-C3B8H10 (Z2a).
Upper part (a) of Fig. 1 shows the typical 11B NMR patterns
of N2a in CDCl3. But when the CDCl3 was removed from the
NMR tube by the stream of nitrogen and the residue re-dissolved
in CD3CN, a completely different spectrum, attributed to the
zwitterionic tautomer Z2a,5 as shown in the bottom part (b) of
Fig. 1, was obtained. Moreover, replacement of CD3CN by CDCl3
in the same sample recovers again the original spectrum of N2a.
These experiments clearly show that a quantitative conversion
between Z2a and N2a tautomers can be achieved via a simple
switch between solvents of different proton-transfer properties.
Both tautomeric forms Z2a and N2a can also be easily isolated in
the solid state as pure chemical individuals and therefore we call
Results and discussion
Syntheses and tautomeric conversions
As shown in Scheme 2, treatment of the C-substituted nido
dicarbadecaboranes 5,6-R¢,R-5,6-C2B8H10 (1) (where R¢,R = H,H
(1a); H,Me, (1b); Me,Me, (1c); H,Ph, (1d) and Ph,Ph, (1e)6 with
proton sponge (PS = 1,8-bis-(dimethylamino)naphthalene) (in situ
generation of anion 1-) and t-BuNC in CH2Cl2, followed by
acidification and evaporation of the CH2Cl2 layer, generated a
series of pure neutral compounds 7-t-BuNH-8,9-R,R¢-nido-7,8,9-
C3B8H9 (N2) (where R,R¢ = H,H (N2a); H,Me (N2b); Me,Me
(N2c); H,Ph (N2d), and Ph,Ph (N2e)), each of which exhibits
tautomerism. Dissolution of these N2 compounds in protic
solvents (PRS), such as MeCN or Me2CO, leads in turn either to
complete (only in the case of the unsubstituted N2a) or equilibrium
(in the case of all other substituted derivatives N2b–N2e, see
also Table 1 below) conversion into the zwitterionic tautomers
7-t-BuNH2-8,9-R,R¢-nido-7,8,9-C3B8H8 (Z2) (where R,R¢ = H,H
(Z2a); H,Me (Z2b); Me,Me (Z2c); H,Ph (Z2d) and Ph,Ph (Z2e)).
Fig.
1
Absolute tautomerism exemplified by entirely different
11B{ H}NMR spectra of the same compound “7-t-BuNH2-7,8,9-C3B8H10”
(2a) at 296 K in (a) CDCl3 and (b) CD3CN. Spectrum (a) corresponds to
the neutral tautomer 7-t-BuNH-7,8,9-C3B8H11 (N2a) and spectrum (b) to
the zwitterionic tautomer 7-t-BuNH2-7,8,9-C3B8H10 (Z2a). Both spectra
are interconvertible by exchange of these two solvents.
1
Scheme
2
Formation and tautomeric conversions between
neutral 7-t-BuNH-8,9-R,R¢-7,8,9-C3B8H9 (N2) and zwitterionic
7-t-BuNH2-8,9-R,R¢-7,8,9-C3B8H8 (Z2) compounds via common anions
[7-t-BuNH-8,9-R2R1-7,8,9-C3B8H8]- (2-). For numbering of the ten- and
eleven-vertex nido cages see structures 1 and Z2, respectively.
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this unique phenomenon absolute tautomerism (AT)4 – the same
compound can be isolated in two different structural forms.
Scheme 2 shows that the N2 and Z2 tautomers differ in
the positioning of one hydrogen atom. This resides either in
the bridging position between the B10 and B11 vertices (N2
tautomers) or on the exohedral nitrogen atom (Z2 tautomers).
From the viewpoint of cluster-boron chemistry such a difference
is rather significant: compounds N2, in which the bridging H-atom
constitutes a part of the cluster, are 7-t-BuNH-derivatives of the
neutral tricarbaborane nido-7,8,9-C3B8H12, whilst compounds Z2
are zwitterionic derivatives of the [nido-7,8,9-C3B8H11]- anion.7
Table 1 demonstrates that the ratio between individual tau-
tomeric forms (defined by KT = CZ2/CN2, where CZ2 and CN2 are
equilibrium concentrations of Z2 and N2 forms in a given solvent1)
is strongly dependent both on the nature of solvent and type of
substitution on cluster carbons. Protic solvents (PRS), such as
CD3CN or (CD3)2CO, induce the formation of the zwitterionic
(Z2) tautomers, while aprotic solvents (APRS), such as CDCl3
and CD2Cl2, induce a complete formation of the neutral (N2)
tautomers in the series of compounds outlined above. Similar
trends of tautomeric conversions were observed also for the
Me and Ph substituted compounds, except that their N2 → Z2
conversions in CD3CN are incomplete. This is shown in Table 1
by the KT constants (defined as KT = CZ2/CN2, where CZ2 and CN2
are equilibrium concentrations of the Z2 and N2 forms in a given
solvent)4 of individual compounds in selected NMR solvents, as
determined by room-temperature 11B NMR spectra. It is seen that
the KT values for the C-substituted derivatives of 2a are strongly
solvent-dependent. For instance, while in CD2Cl2 (or CH2Cl2) the
monomethylated compound 2b occurs only in the neutral N2b
form (KT = 0), in CDCl3, acetone-d6, and CD3CN Z2b ꢀ N2b
equilibria are observed (KT = 0.55, 2.91, and 5.62, respectively).
The dimethylated compound 2c adopts the neutral structure N2c
both in CD2Cl2 and CDCl3, while in CD3CN and (CD3)2CO we
observe Z2c/N2c equilibria (KT = 2.02 and 1.39, respectively).
Similar trends, but different in magnitudes, are observed for the
phenyl analogues 2d and 2e. The KT constants can be thus used
as a measure of proton transfer efficiency, which establishes the
expected order of solvents of decreasing proton-transfer affinity:
MeCN > Me2CO > CHCl3 > CH2Cl2.
Fig.
2
Tautomeric equilibria between Z2 and N2 forms in
CD3CN–CDCl3 solvent mixtures at 296 K.
zwitterionic tautomers Z2 decreases in the series H ꢁ Ph > Me
and also with the number of substituents on the cluster carbon
vertices.
It is reasonable to suggest that the tautomeric conversions
outlined above are achieved via a simple proton transfer between
the two proton donating and proton accepting centers, the N atom
and the open-face B10–B11 bond (see Scheme 2).4 The tautomeric
conversions are thus supposed to proceed via common [7-t-BuNH-
8,9-R,R¢-7,8,9-C3B8H8]- (2-) anions, the protonation of which
at the N center leads to Z2 tautomers, while the protonation
of the open-face B10–B11 bond results in the corresponding
N2 tautomers. The proton transfer from the bridging m-H10,11
position to the N atom is strongly affected by the nature of
substituents (Me and Ph) residing in close proximity of the amine
nitrogen. Both size and electronic effects of these substituents
may play a dominating role in the transfer of the tautomerising
hydrogen. Table 1 clearly shows that the tendency for the formation
of Z tautomers decreases in the following order of substituents:
As a result, pure tautomers N2 were isolated for all compounds
of type 2, while pure zwitterionic tautomers Z2 were isolated
only for the unsubstituted compound Z2a which exhibits absolute
tautomerism. The N2 → Z2 conversions of the C-substituted
compounds are incomplete due to steric and electronic effects of
the Me and Ph substituent(s) residing on the cage carbon atom(s).
These findings are also in agreement with higher stability of the
N2 tautomers, as predicted by RMP2(fc)/6-31G* calculations.4
Nevertheless, these calculations reflect gas-phase results and would
not be expected to exactly reproduce experimental findings from
solutions.
As demonstrated in Fig. 2 for CD3CN–CDCl3 mixtures, the
addition of CD3CN to a CDCl3 solution of N2 leads expectedly
to a classical equilibrium between the Z2 and N2 tautomers.
It is seen that by adding CD3CN to a CDCl3 solution of N2
tautomer the concentration of the Z2 tautomer grows in an
approx. linear manner and that the growth is dependent of the
nature of substituents on the cage-carbon vertices. It can be also
inferred from Fig. 2 that the tendency for the formation of the
8,9-H2 ꢁ 8,9-Ph2 > 8-Ph > 8-Me > 8,9-Me2
Intercomparison of 11B NMR chemical shifts of individual
boron vertices for the whole series of N2 and Z2 tautomeric pairs
is given in Fig. 3 and 4. For C-substituted compounds, the values
for Z2 were extrapolated by subtracting the spectra of N2 from
the mixed spectra. The figure shows sharp, dramatic differences
between individual pairs on one hand and gross similarities within
each series (N or Z) on the other. Similar considerations are also
valid for the corresponding 1H NMR shifts.
Individual tautomers of the N2 and Z2 series can be also isolated
in the solid state, as documented by X-ray diffraction analy-
ses of the zwitterionic 7-t-BuN(Me)H-nido-7,8,9-C3B8H10.COMe2
(Z2configuration)5 and N2b (see Fig. 5). The figure clearly shows
that the two structures differ mainly in the open-face B10–B11
˚
bond distances. This is for N2b, 0.112 A shorter than that in
Z2 because of the presence of the bridging m-H10,11 hydrogen
˚
˚
atom. The C7–N1 distance in N2b (1.486 A) is only 0.023 A
shorter than the equivalent C–N vector in the zwitterionic
4188 | Dalton Trans., 2010, 39, 4186–4190
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Fig. 4 Graphical presentation of experimental data for the Ph-substituted
derivatives of the N2 and Z2 series (NMR data at 296 K in CDCl3 and
CD3CN, respectively). For other comments see Fig. 3.
Experimental
General
All reactions were carried out with use of standard vacuum or
inert-atmosphere techniques as described by Shriver,8 although
some operations, such as column LC (silica gel Aldrich 250-350
mesh), were carried out in air. The starting carboranes of type
1 were prepared according to the literature.6 Dichloromethane
and hexane were dried over CaH2 and freshly distilled before
use. Other chemicals were of reagent or analytical grade and
were used as purchased. Analytical TLC was carried out on
Silufol (silica gel on aluminium foil; detection by I2 vapour,
followed by 2% aqueous AgNO3 spray). Low-resolution mass
spectra were obtained using a Finnigan MAT Magnum ion-
trap quadrupole mass spectrometer equipped with a heated inlet
option, as developed by Spectronex AG, Basel, Switzerland (70 eV,
Fig. 3 Graphical representation of experimental data for the H- and
Me-substituted derivatives of the N2 and Z2 series (NMR data at 296 K in
CDCl3 and CD3CN, respectively). Stick-type intercomparison of 11B NMR
chemical shifts and intensities for individual tautomeric pairs. Included
are also numerical values for individual BH vertices (ordered as d(11B),
1J(BH) and d(1H) (in italics)). Framed (full line) text includes additional
d(1H) data (in italics) and other experimental data are in dotted frames.
For numbering see Scheme 1. Data for Z2a from ref. 5.
7-t-BuN(Me)H-nido-7,8,9-C3B8H10.COMe25 and is therefore con-
sistent with a single bond. However, the C7–N1–C11 angle in N2b
(123.86(8)◦) shows a considerable distortion from the ideal 109◦,
which may be attributed to the presence of a free electron pair
on N1 rather than to the double-bond character of the C7–N1
bond. On the other hand, the associated C7–N1–H1A (109.4(9)◦)
and C11–N1–H1A (111.1(9)◦) angles are in agreement with a
tetragonal arrangement around the N1 atom.
1
EI ionisation). H and 11B NMR spectroscopy was performed at
9.4 T on a Varian Mercury 400 instrument at 296 K. The [11B–11B]-
1
11
10
COSY9 and H–{ B(selective)} NMR experiments were made
essentially as described earlier.11
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placed in their optimised positions and refined as riding atoms
with Uiso(H) assigned to a 1.5 multiple to Ueq of its bonding atom.
The refinement converged (D/smax = 0.001, 202 parameters) to
R = 3.8% for the observed, and wR = 11.0%, GOF = 1.04 for
all diffractions. The final difference map displayed no peaks of
-3
˚
chemical significance (Drmax = 0.23, Drmin = -0.18 e A ).
Conclusions
As far as we are aware, the isolation of pure tautomers of the
unsubstituted compound 2a (structures Z2a and N2a) represents
the first example of absolute tautomerism. This phenomenon
introduces a new type of structural dualism to the discipline of
chemistry, as the same compound can be isolated in two different
structural forms, depending of proton transfer properties of the
solvent used. We have also demonstrated that the C-substituted
Me and Ph derivatives of type 2 can be isolated as pure neutral N2
tautomers which can be partially converted into their zwitterionic
(Z2) counterparts, the tautomeric conversion being remarkably
dependent of the nature of substitution on the cluster carbon
atoms. We are currently studying other chemical consequences of
this unique phenomenon of tautomerism in the area of cluster
boron chemistry.
Fig. 5 ORTEP representation of the crystal and molecular structure of
the neutral tautomer 7-t-BuNH-8-Me-7,8,9-C3B8H10 (N2b). Displacement
ellipsoids are drawn on 50% probability level. Selected intracluster
◦
˚
bond distances (A) and angles ( ) C7–N1 1.486(1), C7–C8 1.5616(12),
C8–C9 1.5234(13), C9–B10 1.6539(16), B10–B11 1.8374(15); C8–C7–B11
107.29(7), C7–C8–C9 115.35(8), C8–C9–B10 110.03(8), C9–B10–B11
102.69(7), B10–B11–C7 104.02(7), C7–N1–C11 123.86(8). Other C–B and
B–B bond lengths fall within usual limits.
General synthesis of the neutral tautomers 7-t-BuNH-
8,9-R,R¢-nido-7,8,9-C3B8H9 (N2) (where R,R¢ = H,H (N2a);
H,Me (N2b); Me,Me (N2c); H,Ph (N2d), and Ph,Ph (N2e)
Acknowledgements
The work was supported by the Ministry of Education of the Czech
Republic (project no. LC 523). I. C. thanks MSM0021620857 for
support.
A solution of compounds of type 1 (1a–1e) (reaction scale ca.
5 mmol) in CH2Cl2 (30 ml) was treated with PS (1.1 g, 5 mmol)
under stirring and cooling at 0 ◦C and the mixture was then stirred
for an additional 2 h at ambient temperature. Upon treatment with
5% aq. HCl (10 ml) under shaking at 0 ◦C, the organic (bottom)
layer was separated, dried with MgSO4 and treated with silica
gel (ca. 5 g). The mixture was evaporated and mounted onto the
top of a silica gel column (2.5 ¥ 30 cm). Elution with CH2Cl2
gave the main fractions of Rf (analytical) ca. 0.3–0.45, which were
evaporated to give 36–55% yields of compounds of type N2 (N2a–
N2e) which were identified by NMR spectroscopy (for properties
of individual compounds see Fig. 3 and 4).
Notes and references
1 See, for example: F. A. Carey and R. A. Sundberg, Advanced Organic
Chemistry, Springer, 5th edn, 2007.
2 N. V. Belova, H. Oberhammer, G. V. Girichev and S. A. Shlykov, J. Phys.
Chem. A, 2007, 111, 2248.
3 R. L. Lindvedt and H. F. Holtzclaw, J. Am. Chem. Soc., 1966, 88, 2713;
A. L. Andeassen, D. Zebelman and S. H. Bauer, J. Am. Chem. Soc.,
1971, 93, 1148.
4 M. Bakardjiev, J. Holub, D. Hnyk, I. C´ısarˇova´, M. G. S. Londesbor-
ˇ
ough, D. S. Perekalin and B. St´ıbr, Angew. Chem., Int. Ed., 2005, 44,
6222–6226.
ˇ
5 B. St´ıbr, J. Holub, I. C´ısarˇova´, F. Teixidor, C. Vin˜as, J. Fusek and Z.
Plza´k, Inorg. Chem., 1996, 35, 3635.
X-Ray crystallography†
ˇ
6 B. St´ıbr, F. Teixidor, C. Vinˇas and J. Fusek, J. Organomet. Chem., 1998,
ˇ
550, 125; Z. Janousˇek, P. Kaszynski, J. D. Kennedy and B. St´ıbr, Collect.
Crystal data for N2b: C8H23B8N, M = 219.75, colourless plate,
0.51 ¥ 0.25 ¥ 0.08 mm3, monoclinic, space group P21/n (no. 14),
Czech. Chem. Commun., 1999, 64, 986.
ˇ
7 J. Holub, B. St´ıbr, D. Hnyk, J. Fusek, I. C´ısarˇova´, F. Teixidor, C. Vinˇas,
˚
a = 9.9813 (2), b = 11.5093 (2), c = 12.7934(2) A, b = 111.6510
Z. Plza´k and P. V. R. Schleyer, J. Am. Chem. Soc., 1997, 119, 7750.
8 D. F. Shriver and M. A. Drezdon, Manipulation of Air Sensitive
Compounds, Wiley, New York, 2nd edn, 1986.
9 See, for example: J. D. Kennedy, in Multinuclear N. M. R., ed. J. Mason,
Plenum Press, New York, 1987, p. 221; W. C. Hutton, T. L. Venable
and R. N. Grimes, J. Am. Chem. Soc., 1984, 106, 29; J. Schraml and
J. M. Bellama, Two-Dimensional NMR Spectroscopy, Wiley, New York,
1982, and references therein.
◦
3
-3
˚
(11) ; V = 1365.99(4) A , Z = 4, Dc = 1.069 Mg m . Diffraction
data were recorded with a Nonius Kappa CCD diffractometer
˚
(graphite-monochromatised Mo Ka radiation, l = 0.71073 A at
150(2) K; absorption was neglected (m = 0.05 mm-1). A total of
24 777 was measured (qmax = 27.5◦), from which 3120 were unique
(Rint = 3.0%), and 2746 observed according to the I > 2s(I)
criterion.
10 X. L. R. Fontaine and J. D. Kennedy, J. Chem. Soc., Dalton Trans.,
1987, 1573.
The structure was solved by direct methods (SIR92)12
and refined by full-matrix least squares routine based on
F2 (SHELXL9713). Non-hydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms were
refined isotropically, except those in methyl moieties which were
ˇ
11 B. St´ıbr, J. Holub, M. Bakardjiev, D. Hnyk, O. Tok, W. Milius and B.
Wrackmeyer, Eur. J. Inorg. Chem., 2002, 2320.
12 A. Altomare, G. Cascarano, C. Giacovazzo and A. Guagliardi, J. Appl.
Crystallogr., 1993, 23, 350.
13 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
64, 112.
4190 | Dalton Trans., 2010, 39, 4186–4190
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